Far beyond the orbit of Pluto exists a celestial body that’s a little smaller, a little colder, and a little denser—the dwarf planet Eris. In Greek mythology, Eris is the goddess of strife, and never was there a more appropriately named body in the solar system. When astronomer Mike Brown of Caltech and his team discovered Eris in 2005, the finding set off a chain reaction that would see the planetary status of Pluto called into question. Here are 10 things you might not know about Eris.
If you were an astronaut, you wouldn’t find an Eridian day, at 25.9 hours, too disconcerting. This compares favorably with, say, Venus, whose day lasts 5832 hours (admittedly, it's an outlier). An Eridian year is a bit longer than what we're used to, with the dwarf planet completing an orbit of the Sun every 557 Earth years. And that orbit is not along the relatively flat plane with the orbits of most of the other planets of the solar system. Imagine your elementary school solar system model of planets on wires around a light bulb: Instead of a path neatly aligned with the other planets, Eris’s orbit is tilted at a 44 degree angle.
After Eris’s discovery, the best measurements then available placed it as slightly larger than Pluto, with a radius of 722 miles. But after the initial spacecraft reconnaissance of Pluto by New Horizons in 2015, Pluto’s ranking as the ninth-largest planetary object orbiting the Sun was restored; it is now known to have a radius of 736 miles. In comparison, Earth’s Moon has a radius of 1079 miles. Ganymede, Callisto, Io, and Europa (Jupiter’s largest moons), Titan (Saturn’s largest moon), and Triton (Neptune’s largest moon) are also bigger than Pluto. On the other hand, Eris is 34 percent denser than Pluto.
When Brown’s team discovered Eris, it was initially hailed as either the 10th planet of the solar system, or a big problem for scientists who like nicely ordered celestial objects. The discovery of Eris came on the heels of the discoveries of Sedna and Quaoar, both beyond the orbit of Neptune. Astronomers were looking at the possibility of a dozen planets in the solar system or more, because—based on these three—who knew how many Pluto-sized bodies were out there? The International Astronomical Union eventually defined a planet in our solar system as something that has achieved hydrostatic equilibrium (in other words, it's round), orbits the Sun, and has “cleared its neighborhood” (i.e., is gravitationally dominant in its orbit). Yet the debate continues [PDF].
Eris has a moon called Dysnomia that circles the dwarf planet every 16 days. In Greek mythology, Dysnomia is the name of one of Eris’s daughters and means “anarchy.”
Before it was called Eris, it was called 2003 UB313 (a provisional designation by the International Astronomical Union). But before that, Brown’s team of astronomers named it Xena—yes, of Warrior Princess fame. “We always wanted to name something Xena,” Brown told The New York Times in 2005 after the discovery. Among Brown’s colleagues, Dysnomia was called Gabrielle, who was, of course, Xena’s sidekick.
The primary way to analyze the composition of the surface of a celestial body is through spectroscopy, which is basically looking at an object and seeing how much light comes back at you as a function of wavelength. Many materials have characteristic absorptions of light at certain frequencies, and so less light will come back to you at that frequency.
“Eris has very, very strong methane ice absorption bands,” Will Grundy, a planetary scientist at Lowell Observatory and a member of the New Horizons team, tells Mental Floss. “They are much stronger than Pluto’s, and of course we’ve seen methane all over the place on Pluto, so I think it’ll be more ubiquitous on Eris’s surface.” The implication is that Eris is more than just a dead ice rock in space, because methane degrades very quickly in a space environment, darkening and forming heavier hydrocarbons. “The fact that it’s bright and covered with methane ice says it’s refreshing its surface relatively rapidly, and there are any number of ways it can do that. One is the methane just periodically sublimates underneath the atmosphere and then re-condenses somewhere else, just sort of painting on top of whatever dark stuff that forms,” Grundy says.
Pluto data returned from the New Horizons spacecraft give scientists new ideas about the processes that might be at work on Eris. “One of the things the Pluto flyby showed us that nobody really talked about, even in wild speculations, was something like Sputnik Planitia: this big, bright, teardrop-shaped region on the encounter hemisphere. Volatile ices there are trapped in a deep basin and they are just convectively overturning, like a simmering pot of soup,” Grundy says.
That process might be happening writ large on Eris. It might be, in a sense, an ice lava lamp planet. “I’ve called it a Sputnik planet,” Grundy says, “but nature is much more clever than scientists at coming up with new ways of doing things with the same old ingredients. Who knows, we might get there and find out it’s doing something completely different than Pluto was doing to refresh its surface. The real lesson is that activity on a lot of different timescales is possible, even on a tiny little planet that’s at frigid temperatures, far away from the sun.”
In comparative planetology, scientists use planets to understand other planets. By studying Venus, which is similar to Earth in terms of size, mass, and basic composition, scientists can better understand how our planet operates and evolved. The objects in Eris’s celestial neighborhood work the same way. “The Kuiper Belt”—a region rich in rocky and icy objects beyond Neptune’s orbit—“is an incredibly rich environment for comparative planetology because there are so just many of these tiny planets out there,” Grundy says. “It’s going to take a while to discover them all, let alone explore them all, but that’s what is exciting about it.” The New Horizons data from Pluto are helping planetary scientists develop models to tease out the secrets of Eris.
“If you work out the surface area of, say, objects there that are bigger than 100 kilometers, based on extrapolation, the Kuiper Belt has more solid geology surface area than of all of the planets in the solar system—including the terrestrial planets—combined,” Grundy explains, adding that it holds true even if you wanted to include the ocean floor on Earth. “If you like geology—and especially if you like exotic, cryogenic temperature geology—this is the place to explore, and there’s just so much territory to explore out there.”
It took New Horizons, one of the fastest spacecrafts ever built, nine years to get to Pluto. Eris is currently three times farther from the Sun than Pluto (though due to a highly elliptical orbit, this number changes), so if a mission is ever approved, don’t expect to find out how it all ends. “It takes decades to pull something like that together, so if you want to be around to see the results, you’ve got to start young,” Grundy says. A possible future Kuiper Belt mission might be part of a flyby mission to Uranus or Neptune, after which the spacecraft would continue into that region of space. It will be a very long time before technology allows an Earth-centric telescope—in space or otherwise—to take pictures of the geology of Eris.